I have GE refrigerator M # GSS22JFMDCC, evaporator is frosting all the way through it, but top half of evaporator coil has excessive frost and some ice. This does not look like a normal defrost problem. I bypassed board with jumper wire and defrost heater came on which usually indicates a bad sensor. But with the excessive frost and ice on top half of evaporator I am not 100 percent sure. Does this sounds like a sealed system problem? Can anyone help me. Thank You

Sounds like a classic case of a bad evaporator thermistor (clipped to the evaporator) causing incomplete defrosts. It gets water in it which raises its resistance and makes the muthaboard think it should stop defrosting too early. Check the resistance of that thermistor with it dunked in a glass of ice water, should get around 16 k-ohms. Measure the resistance from its wire harness connector back on the muthaboard.